The present invention relates to travelling wire EDM apparatus and method, in general, and more particularly to an apparatus and method for holding a portion of the workpiece severed from the remaining of the workpiece at the end of a cutting operation.
While severing a portion from a workpiece blank in the course of effecting a closed loop cut in the workpiece blank by an electrode wire in a travelling wire EDM apparatus, for example when cutting an opening in a die or when cutting a punch, the weight of the portion of the workpiece material situated in the interior of a closed loop cutting path causes substantial internal stresses in the workpiece towards the end of the cut, which in turn causes machining defects. In addition, the sudden fall of the cut-off portion at the end of the cutting operation may cause substantial damages, and it is desirable to prevent the fall of the cut-off portion. Diverse holding fixtures have been designed in the past to solve the problem, for example in the form of a removable support disposed below the portion cut-off from the workpiece blank, or in the form of a stationary support provided with a slot through which the electrode wire is passed and on which slides the workpiece blank in a direction perpendicular to the axis of the slit, or yet taking the form of a plurality of movable support arms engaged under the workpiece blank and driven in rotation during the cutting operation. The first and third of the solutions hereinabove mentioned solve only partially the problem while the second solution presents the inconvenience of requiring a sliding support which must support the workpiece blank with a high accuracy, which in turn requires a cumbersome and expensive structure together with the requirement that the lower surface of the workpiece blank be strictly in a single plane.